The transmission capacity of optical communication systems is limited by the spectral range of the optical fiber. One way to increase spectral efficiency is to multiplex data streams data using different polarizations of light. However, due to random birefringence in the optical fiber, signals carried in different polarizations experience polarization mixing or even polarization mode dispersion (PMD). This requires polarization demultiplexing or PMD compensation in the receiver in order to correctly recover the transmitted signals.